The present invention relates generally to method of bonding metals and non-metals using inductive heating. The method is advantageously applicable to the bonding of silicon to steel, such as for placing silicon MEMS strain sensors on steel mechanical components, such as bearing races.
Micro-electromechanical systems (MEMS) is a technology encompassing the integration of mechanical elements, such as structures and actuators, with electronics on a common silicon substrate through micro-fabrication technology. The electronics are fabricated using well established semiconductor processing process techniques (e.g., metal oxide silicon (e.g., NMOS, PMOS and CMOS processes)), and the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
Advancements in the MEMS field have seen the development of realizable complex devices and sensors. MEMS sensors, such as accelerometers, gas sensors, pressure sensors, and others have become so refined and reliable that their detection abilities can function, in theory, while bonded to and sensing a wide range of intricately engineered components. In reality, however, to integrate these sensors with typical manufactured parts it becomes critical to re-examine the limitations of conventional bonding techniques capable of adhering silicon, which is used as a substrate for the vast majority of micro-fabricated sensors, to a wide range of manufactured components made of, among other metals, steel. More importantly, a successful silicon-to-steel bonding process could lead to the deployment of MEMS sensors for various applications, including the health monitoring of key mechanical components in a wide range of industries.
For a device bond to be effective, it should possess certain characteristics that allow it to remain bonded to a component's surface. The bond strength should possess good mechanical characteristics, including high tensile, compressive, and fatigue strength, resistance to creep, and corrosion resistance. For example, the bond should preferably be able to withstand years in harsh, oily environments for applications in machine-related components. Furthermore, the stresses induced in the bonding material during the bonding process should be manageable, and damage to the manufactured component should be minimal. In addition, if bonded sensors are to be easily incorporated into production of these manufactured components, the bonding process should be rapid and repeatable.
Steel is used for a wide range of manufactured components, and the technologies utilized in bonding materials to steel are numerous and well developed. However, many of these techniques possess intrinsic limitations that prevent them from being utilized in bonding silicon sensors to steel. Polymeric adhesives, such as epoxies, for example, offer an inexpensive and simple approach to bonding silicon to steel, but their bonds typically exhibit long-term creep, poor corrosion resistance, and require excessive bonding times. Diffusion bonding is another method that can create a very strong bond. But the times required for the bonding process can range from minutes to hours, and the bonding temperatures can be very high. Brazing and welding operations form very strong bonds, but the temperatures are well above the point at which the tempering of high-hardness steel begins to change. Laser welding and microwave heating have been shown to work and provide localized heating in a rapid period of time, but they both require expensive equipment setups, with microwave heating necessitating a vacuum chamber. Also, micro-heaters have been demonstrated to deliver a high amount of heat to a localized region, but such features require additional device fabrication processes. Another possible bond technique is ultrasonic bonding because it is a rapid, low temperature process; however, the vibration inherent in the process could damage the MEMS structures.
Alternative techniques for advancing this technology would be desirable.